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Synlett 2024; 35(17): 1978-1984
DOI: 10.1055/a-2298-0282
letter
Energetic Molecules

Introduction of an N-Amino Group onto 4-(Tetrazol-5-yl)-5-nitro-1,2,3-triazole: A Strategy for Enhancing the Density and Performance of Energetic Materials

Xun Huang
a   Key Laboratory of Fluorine and Nitrogen Chemistry and ­Advanced Materials, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai, 200032, P. R. of China
,
Pinxu Zhao
a   Key Laboratory of Fluorine and Nitrogen Chemistry and ­Advanced Materials, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai, 200032, P. R. of China
b   School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei Road 200, Nanjing, 210094, Jiangsu, P. R. of China
,
Haifeng Huang∗
a   Key Laboratory of Fluorine and Nitrogen Chemistry and ­Advanced Materials, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai, 200032, P. R. of China
,
Jun Yang∗
a   Key Laboratory of Fluorine and Nitrogen Chemistry and ­Advanced Materials, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai, 200032, P. R. of China
› Author Affiliations

This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant No. 22175196), the CAS Project for Young Scientists in Basic Research (Grant No.YSBR-052), the Shanghai Science and Technology Committee (20XD1404800) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB0590000).
› Further Information
 
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Abstract

2-Amino-5-nitro-4-(tetrazol-5-yl)-1,2,3-triazole (HANTT), its corresponding energetic salts and a dimeric azo compound are successfully synthesized. Compared to 5-nitro-4-(tetrazol-5-yl)-1,2,3-triazole (H2NTT), the neutral N-amino compound HANTT exhibits excellent properties in many aspects, including a higher density (ρ = 1.86 g cm–3), a better detonation performance (D v = 8931 m s–1, P = 32.2 GPa) and a higher thermal decomposition temperature (T d = 237 °C). Among the prepared materials, the hydroxylammonium energetic salt exhibits the best detonation performance (D v = 9096 m s–1, P = 32.8 GPa) and an acceptable mechanical sensitivity (IS = 12 J, FS = 144 N). HANTT, the energetic salts and the azo compound are fully characterized by infrared spectroscopy, multinuclear NMR spectroscopy, elemental analysis and differential scanning calorimetry.


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Key words

energetic materials - 1,2,3-triazole - N-amino - hydrogen bond - detonation performance

High-energy density materials (HEDMs) are a class of energetic materials characterized by their high density and high detonation performance, making them increasingly crucial in military weaponry and civil-blasting applications.[1] [2] [3] [4] [5] [6] To further enhance national defense capabilities and ensure security, there is an urgent need to develop energetic materials with better performances. In recent years, scientists have successfully synthesized various nitrogen-rich energetic compounds with excellent density and detonation properties. Typically, HEDMs are expected to exhibit sensitivities comparable to those of trinitrobenzene (TATB) or trinitrotoluene (TNT), while possessing detonation properties greater than those of HMX or CL-20. However, most energetic compounds face a trade-off between high energy and safety due to the reduction in stability caused by the introduction of more energetic groups.[7–12] Additionally, considerations such as costs, yields, simple synthetic processes, and environmental friendliness should also be taken into account.[13–18]

With three catenated nitrogen atoms, the 1,2,3-triazole ring (240 kJ mol–1) has a high heat of formation compared to furoxan (51–185 kJ mol–1), and higher than that of the isomeric 1,2,4-triazole ring (182 kJ mol–1). However, some 2H-1,2,3-triazole energetic compounds have low densities, high sensitivities, low thermal stabilities or inadequate detonation performances.[19] [20] [21] Increasing the number and strength of hydrogen bonds (HBs) and van der Waals forces can significantly enhance the overall stability of energetic compounds, representing a key approach to improving their stability. The impact of adding a more robust hydrogen bond system compared to a weaker van der Waals force would be more pronounced. Therefore, it is crucial to find an energetic group with abundant potential for hydrogen bonds to provide sufficient stability and additional energy for the entire energetic compound.[22] [23] Given this background, it is worthwhile exploring 1,2,3-triazole-based nitrogen-rich energetic compounds with an extra N-amino group (N-NH2). Such N-amino energetic compounds may exhibit enhanced characteristics, for example, increased densities, elevated thermal stabilities, improved detonation performances, and reduced sensitivities.

Bicyclic nitrogen-rich energetic compounds tend to show good properties (e.g., higher thermal stability, lower mechanical sensitivity) and are considered as potential alternatives to monocyclic energetic compounds due to the larger conjugate system and higher heat of formation of the nitrogen-rich skeleton of bicyclic compounds.[24] [25] [26] [27] [28] However, some bicyclic energetic compounds with tetrazole rings, such as 4,5-bis(tetrazol-5-yl)-1,2,3-triazole (H3BTT) and 5-nitro-4-(tetrazol-5-yl)-1,2,3-triazole (H2NTT), have low densities, low thermal stabilities and high sensitivities, which are unfavorable for their transportation, use, and storage.[29] [30] Recently our studies revealed that the introduction of an N-amino group onto a 1,2,3-triazole energetic framework is helpful for improving various properties of these energetic compounds.[31] [32] For example, the introduction of an N-amino group onto H3BTT gave 2-amino-4,5-bis(tetrazol-5-yl)-1,2,3-triazole (H2ABTT) and improved the comprehensive properties of this energetic compound, with a higher density, a higher decomposition temperature, a higher detonation performance and a lower sensitivity being observed.[33] Therefore, we planned to use the same synthetic strategy to introduce an N-amino group onto H2NTT. As shown in Figure [1], the introduction of an N-amino group improves the density, thermal stability, and detonation properties of the product, 2-amino-5-nitro-4-(tetrazol-5-yl)-1,2,3-triazole (HANTT) (4), compared to H2NTT.

Zoom Image
Figure 1 (a) N-NH2-1,2,3-triazole design strategy. (b) Previous work. (c) This work.

In this work, the N-amino group was introduced onto H2NTT by utilizing the powerful amination reagent O-tosylhydroxylamine (THA).[34] In addition, the energetic salts 5–7 and the azo compound 8 were synthesized (Scheme [1]). These products were fully characterized by infrared spectroscopy, multinuclear NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). The structures of compounds 4–8 were confirmed by single-crystal X-ray diffraction analyses.

Zoom Image
Scheme 1 Synthetic route towards products 4–8

The starting material 4-cyano-5-amino-1H-1,2,3-triazole (1) was readily synthesized according to the literature.[35] Its ammonium salt (2) was prepared by oxidation of compound 1 with 30% H2O2 and 98%H2SO4. Subsequent treatment of compound 2 with the fresh amination reagent THA led to compound 3; the amination reagent THA was readily synthesized following established procedures in the literature.[40] The reaction of compound 3 with NaN3 and ZnCl2 via a 1,3-dipolar cycloaddition yielded compound 4. Next, compound 4 was treated with three equivalents of a nitrogen-rich base (ammonium hydroxide, hydroxylamine or hydrazine hydrate) in ethanol to afford the three energetic salts 5–7. In addition, a diazotization reaction was carried out using compound 4 in KMnO4 and HCl to afford product 8 (Scheme [1]).

Single crystals of compounds 4–7 and 8·2H2O were obtained by slowly evaporating the solvent from the suitable saturated solutions at room temperature. The crystallographic data and the details regarding structural determinations are given in Table S1 of the Supporting Information. Detailed information encompassing bond lengths and bond angles are also provided in the Supporting Information.

Crystals of compound 4 suitable for single-crystal X-ray diffraction were obtained by evaporation from a methanol solution at room temperature. Compound 4 crystallizes in the monoclinic P21 space group with four formula units in each unit cell. The calculated density is 1.840 g cm–3 at 213 K. The crystal structure is illustrated in Figure [2a1]. The N-NH2 group N(6)–N(8) in 4 exhibited an N–N bond length of 1.3894 Å, which is between the lengths of a single and a double N–N bond. The NO2 groups between two adjacent molecules are close together and the nearest O–O distance is 2.71 Å, which is much smaller than the value (3.04 Å) of the van der Waals O–O bond length; this may result in compound 4 being more sensitive. Compound 4 exhibited a dihedral angle of 27.8° between the triazole ring and the tetrazole ring (Figure [2a2]). The NO2 group and the triazole ring are almost planar, with an O(4)–N(18)–C(6)–C(5) dihedral angle of 4.9°. In addition, extensive intramolecular and intermolecular hydrogen bonds were observed (Figure [2a3]); for instance, N(1)–H(1).N(4), N(1)–H(1).O(1), N(10)–H(10).N(13), N(10)–H(10).O(4), etc., whilst the bond length ranges between 2.07 and 2.69 Å. The crystals of compound 4, as shown in Figure [2a4], exhibited a wavelike crystal stacking mode with an intermolecular layer spacing of 2.90 Å. Compared with typical aromatic π–π interactions (3.65–4.00 Å), the shorter molecular interlayer distance will form a stronger intermolecular π–π interaction, which is beneficial for improving the overall stability of the molecule.

Zoom Image
Figure 2 Crystal structures and packing diagrams of compounds 4 [(a1)–(a4)], 5 [(b1)–(b4)], 6 [(c1)–(c4)], 7 [(d1)–(d4)] and 8·2H2O [(e1)–(e4)].

Crystals of compound 5 suitable for single-crystal X-ray diffraction were obtained by evaporation from a methanol solution at room temperature. Compound 5 crystallized in the triclinic P-1 space group with two formula units in each unit cell. The calculated density is 1.582 g cm–3 at 293 K. The crystal structure is illustrated in Figure [2b1]. The N-NH2 group N(6)–N(8) in 5 exhibited an N–N bond length of 1.3881 Å, which is between the lengths of a single and a double N–N bond. Due to the absence of N–H on the tetrazole ring, there was no hydrogen bond between the tetrazole ring and the nitro group, resulting in a better planarity of the tetrazole ring and the triazole ring, hence the dihedral angle between the triazole and tetrazole rings is reduced to 17.8° (Figure [2b2]), and the NO2 group and triazole ring were almost planar. In addition, extensive intramolecular and intermolecular hydrogen bonds were observed (Figure [2b3]).

Crystals suitable for single-crystal X-ray diffraction of compound 6 were obtained by evaporation from an ethanol solution at room temperature. It crystallized in the monoclinic P21/n space group with four formula units in each unit cell. The calculated density is 1.790 g cm–3 at 293 K. The crystal structure is illustrated in Figure [2c1]. The N-NH2 group N(2)–N(4) in 6 exhibited an N–N bond length of 1.3898 Å, which again is between the lengths of a single and a double N–N bond. Due to the absence of N–H on the tetrazole ring, there was no hydrogen bond between the tetrazole ring and the nitro group, resulting in a better planarity of the tetrazole and triazole rings; as such, the dihedral angle between the triazole and tetrazole rings was reduced to 13.8° (Figure [2c2]), and the NO2 group and triazole ring were almost planar. In addition, extensive intramolecular and intermolecular hydrogen bonds were observed (Figure [2c3]).

Crystals of compound 7 suitable for single-crystal X-ray diffraction were obtained by evaporation from a methanol solution at room temperature. It crystallized in the monoclinic P21/c space group with four formula units in each unit cell. The calculated density is 1.701 g cm–3 at 293 K. The crystal structure is illustrated in Figure [2d1]. The N-NH2 group N(2)-N(4) in 7 exhibited a bond length of 1.3862 Å, which is between the lengths of a single and a double N–N bond. Due to the absence of N–H on the tetrazole ring, there was no hydrogen bond between the tetrazole ring and the nitro group, resulting in a better planarity of the tetrazole and triazole rings, hence the dihedral angle between the triazole and tetrazole ring was reduced to 13.9° (Figure [2d2]), and the NO2 group and triazole ring were almost planar. In addition, extensive intramolecular and intermolecular hydrogen bonds were observed (Figure [2d3]).

Crystals suitable for single-crystal X-ray diffraction of the dihydrate 8·2H2O were obtained by evaporation from a methanol solution at room temperature. It crystallized in the monoclinic P21/n space group with two formula units in each unit cell. The calculated density is 1.795 g cm–3 at 293 K. The crystal structure is illustrated in Figure [2e1]. The bond lengths N(2)–N(4) are 1.391 Å, which is between the length of a single and a double N–N bond. The azo bond was almost coplanar with the triazole ring, as indicated by the torsion angles of N(1)–N(2)–N(4)–N(4′) of –4.0°, whilst the NO2 group and triazole ring were almost planar, as indicated by the torsion angles of O(1)–N(9)–C(1)–N(1) of –4.9°. Additionally, the N–H of the tetrazole ring had two hydrogen bonds: N(8)–H(8).O(3) and N(8)–H(8).O(2), so the dihedral angle of the tetrazole ring and the triazole ring was 27.8°. The crystal of compound 8·2H2O exhibited a wavelike stacking mode with an intermolecular layer spacing of 2.51 Å (Figure [2e4]). Compared with the typical aromatic π–π interactions (3.65–4.00 Å), the shorter molecular interlayer distance will form a stronger intermolecular π–π interaction, which is beneficial in improving the overall stability of the molecule. Such a short layer spacing allowed the density of compound 8·2H2O to approach 1.8 g cm–3, even though it contained two molecules of water of crystallization.

Thermogravimetric analysis-differential scanning calorimetry (TG-DSC) was performed to investigate the thermal properties of compounds 4–7 and 8·2H2O at a heating rate of 5 K min–1 under a nitrogen atmosphere. The DSC curves are shown in Figures S22–S26 (see the Supporting Information). As shown in Table [1], compounds 4–7 had decomposition temperatures (onset) of 237 °C or above, and exhibited higher thermal stabilities than that of the conventional secondary explosive RDX (T d = 284 °C). Additionally, as expected, the introduction of an amino group improved the thermal stabilities of the energetic materials, with decomposition temperatures increasing from 188 °C (H2NTT) to 237 °C (compound 4). However, the decomposition temperature of the azo compound 8·2H2O (T d = 170 °C) was significantly lower than that of the N-amino compound 4. In terms of mechanical sensitivity, the impact sensitivities (IS) and friction sensitivities (FS) of the target compounds were measured using BAM methods.[37] The IS and FS values of the N-NH2 compound 4 were 2 J and 64 N, respectively, which are more sensitive than those of H2NTT (IS = 4 J, FS = 108 N). Energetic salts 5–7 displayed lower sensitivity than the neutral compound 4, with IS and FS values ranging from 6–12 J and 128–180 N, respectively (Table [1]). Finally, the IS and FS values of the dihydrate azo compound 8·2H2O were 1 J and 30 N, respectively, which suggests that it is very sensitive.

To further investigate the relationships between the structures and properties of compounds 4–8 prepared in this study, the Hirshfeld surfaces and two-dimensional (2D) fingerprints of these compounds (Figures [3a–c]) were analyzed using the tool suite in the program Crystal Explorer.[38] [39] On the Hirshfeld surfaces, the red regions with higher electron density represent strong interactions, the white regions with moderate electron density represent slightly weaker interactions, and the blue regions with lower electron density represent no significant interactions (Figure [3a]). From the fingerprint plots of 4–7 and 8·2H2O, it was observed that two clear sharp spikes exist in the bottom left corners of the plots, which indicate interactions of O…H and N…H (Figure [3b]). The contribution percents of individual atomic contacts on the Hirshfeld surfaces of 4–7 and 8·2H2O are shown in Figure [3c]. According to Figure [3c], hydrogen-bonding interactions (O…H and N…H) play a dominant role, contributing 47.3%, 63.1%, 61.2%, 62.8% and 42.6% of the total interactions in 4–7 and 8·2H2O, respectively. Meanwhile, the π–π interactions (C…N, N…N, C…O and N…O) accounted for approximately 45.9%, 27.8%, 29.1%, 28.2% and 54.8% of the total interactions in 4–7 and 8·2H2O, respectively. The percentage of N–H and O–H interactions of salts 5–7 (61.2–63.1%) exceed those of compound 4 (47.3%), indicating that the greater numbers of hydrogen bonds in 5–7 lead to their low mechanical sensitivity. Overall, more hydrogen bond interactions may contribute to the insensitivity of energetic compounds.

Zoom Image
Figure 3 (a) Hirshfeld surfaces of 4–7 and 8·2H2O. (b) Two-dimensional fingerprint plots in the crystal packings of 4–7 and 8·2H2O. (c) Contribution percentages of individual atomic contacts to the Hirshfeld surfaces of 4–7 and 8·2H2O.

The physicochemical and detonation properties of the energetic compounds prepared in this study are summarized in Table [1]. Densities (ρ) were measured at room temperature by using a gas pycnometer, with the measured values ranging between 1.58 and 1.86 g cm–3. Among the synthesized derivatives, the neutral compound 4 had the highest density of 1.86 g cm–3, which is also greater than that of RDX (1.80 g cm–3). The salts 5–7 showed comparatively lower densities (5: 1.58 g cm–3, 6: 1.78 g cm–3, 7: 1.70 g cm–3) with respect to the neutral compound 4 (1.86 g cm–3), whilst compound 8·2H2O had a high density of 1.81 g cm–3. The heats of formation (ΔH f) of all the studied compounds were calculated by using the Gaussian 09 program suite based on isodesmic reactions (see the Supporting Information).[40] All the newly synthesized compounds 4–7 and 8·2H2O were found to have positive ΔH f values, ranging from 589.4 to 858.4 kJ mol–1, which exceeded those of TATB (–140.0 kJ mol–1) and RDX (–70 kJ mol–1). The neutral compound 4, with an additional N-amino group, exhibited a high heat of formation value of 589.4 kJ mol–1/2.99 kJ g–1, which is higher than that of H2NTT (516.0 kJ mol–1/2.83 kJ g–1). Based on experimental densities (ρ) and calculated heats of formation (ΔH f), the detonation properties were calculated using EXPLO5 (version 6.06). The detonation velocities (D v) of the studied compounds ranged between 8053 and 9096 m s–1, and the detonation pressures (P) ranged between 23.5 and 32.2 GPa. Due to the increase of density and heat of formation, the N-NH2 compound 4 exhibited significantly better detonation properties (D v = 8931 m s–1, P = 32.2 GPa) compared to H2NTT (D v = 8251 m s–1, P = 26.3 GPa). Due to an improvement in the oxygen balance as well as the nitrogen content, the hydroxylammonium salt 6 had the best detonation properties (D v = 9096 m s–1, P = 32.8 GPa) among all the synthesized compounds, being close to those of RDX (D v = 8795 m s–1, P = 34.9 GPa).

Table 1 Physicochemical and Detonation Properties of Energetic Compounds 4–7 and 8·2H2O

Compound

ρ a

T d b

ΔH f c

P d

D v e

IS f

FS g

N h

OB i

4

1.86

237

589.4/2.99

32.2

8931

   2

  64

63.95

–44.6

5

1.58

239

599.5/2.80

23.5

8053

  12

 180

65.41

–52.3

6

1.78

237

635.9/2.76

32.8

9096

  12

 144

60.86

–41.7

7

1.70

238

742.5/3.24

29.7

8909

   6

 128

67.23

–52.4

8·2H2O

1.81

170

858.4/2.01

29.2

8524

   1

  30

59.15

–33.8

H2NTT [30]

1.69

188

516.0/2.83

26.3

8251

   4

 108

61.53

–43.9

H2ABTT [34]

1.86

303

1019.3/4.63

31.7

9185

>100

>360

76.35

–74.1

H3BTT [31]

1.69

277

801.0/3.90

24.8

8360

   2

 240

75.11

–72.7

TATB [18]

1.94

324

–140.0/–0.53

31.2

8114

  50

>360

32.55

–55.8

RDX [18]

1.80

204

70.3/0.32

34.9

8795

   7.4

 120

37.81

–21.6

a Density measured at room temperature using a gas pycnometer (g cm–3).

b Decomposition temperature (onset) analyzed under an N2 atmosphere (5 K min–1).

c Heat of formation (kJ mol–1/kJ g–1).

d Detonation pressure calculated using EXPLO5 V6.06 (GPa).

e Detonation velocity calculated with EXPLO5 V6.06 (m s–1).

f Impact sensitivity (J).

g Friction sensitivity (N).

h Nitrogen content (%).

i Oxygen balance (OB) based on CO2. For a compound with a molecular formula of CaHbNcOd, ΩCO2 (%) = 1600 [(d – 2a – b/2)/MW]; MW, molecular weight (%).

In summary, a series of energetic compounds of N-NH2-1,2,3-triazole have been synthesized. The crystal structures of products 4, 5, 6, 7 and 8·2H2O were confirmed by X-ray diffraction analysis. All the compounds exhibited good detonation properties (D v: 8053–9096 m s–1; P: 23.5–32.8 GPa) and densities (ρ: 1.58–1.86 g cm–3). The N-amino neutral compound HANTT (4) possesses a better detonation performance (D v = 8931 m s–1, P = 32.2 GPa) and a higher density (1.86 g cm–3) than RDX, and thus has potential as a next-generation secondary explosive. Moreover, the high mechanical sensitivity of HANTT was explained by theoretical analysis of the Hirshfeld surface and fingerprint plots. Among the synthesized compounds, the hydroxylammonium salt 6 exhibited the best detonation performance, with a detonation velocity of 9096 m s–1 and a detonation pressure of 32.8 GPa, suggesting that it has considerable potential for application as a HEDM. Consequently, this is the second successful construction of N-NH2-1,2,3-triazole nitrogen-rich energetic compounds with high density and high detonation performance via the introduction of an N-amino group, and provides an efficient strategy for the construction of high-performance energetic compounds.


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Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2298-0282.
  • Supporting Information

Corresponding Authors

Haifeng Huang
CAS Key Laboratory of Energy Regulation Materials, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences
Shanghai, 200032
P. R. of China   

Jun Yang
CAS Key Laboratory of Energy Regulation Materials, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences
Shanghai, 200032
P. R. of China   

Publication History

Received: 27 February 2024

Accepted after revision: 02 April 2024

Accepted Manuscript online:
02 April 2024

Article published online:
24 April 2024

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Zoom Image
Figure 1 (a) N-NH2-1,2,3-triazole design strategy. (b) Previous work. (c) This work.
Zoom Image
Scheme 1 Synthetic route towards products 4–8
Zoom Image
Figure 2 Crystal structures and packing diagrams of compounds 4 [(a1)–(a4)], 5 [(b1)–(b4)], 6 [(c1)–(c4)], 7 [(d1)–(d4)] and 8·2H2O [(e1)–(e4)].
Zoom Image
Figure 3 (a) Hirshfeld surfaces of 4–7 and 8·2H2O. (b) Two-dimensional fingerprint plots in the crystal packings of 4–7 and 8·2H2O. (c) Contribution percentages of individual atomic contacts to the Hirshfeld surfaces of 4–7 and 8·2H2O.